Films of thermoplastic polymer compositions have found many commercial uses, primarily in packaging. One large field of use of such films is in the production of plastic bags, such as trash and leaf bags, and in shipping sacks.
A plastic film suitable for use in the fabrication of bags, shipping sacks and other items must possess a variety of good physical and mechanical properties. The film should be strongly resistant to tearing and exhibit good tensile strength. Clarity or transparency is another desired property. It is advantageous if heat sealing of layers of the film may be conducted over a broad range of sealing temperatures while obtaining good seal strength. To be favorable economically, the film should be readily processable at high production speeds (line speeds) and an increase in film gauge should not be required to maintain satisfactory physical and mechanical properties.
Films for use in packaging and sacking are conventionally produced by the well known blown-bubble extrusion process. The process is briefly described, for example, in "Encyclopedia of Chemical Technology," Kirk-Othmer, 2nd Ed., 1966, Vol. 9, pp. 235-236. In that process, a tube of thermoplastic polymer melt is extruded through an annular die, inflated by air to a size dictated by the film properties desired and limited by polymer melt properties, cooled with externally blown air (typically about 40.degree. to 60.degree. F.), collapsed to a flat tube, and wound on rolls. The ratio of bubble diameter to die diameter known as the blowup ratio, determines the extent to which the resulting film is biaxially oriented, which, together with the tensile properties of the polymer and the thickness (gauge) of the film determines the strength of the resulting film. The production of blown film from commercial film grade polybutylene resins is described in Technical Bulletin SC:397-79 of Shell Chemical Company, entitled "Processing Shell Polybutylene Film Grade Resins," published May, 1979. Conventional nomenclature frequently refers to films of greater than 10 mil thickness as sheet, although the process for producing them is called blown film extrusion, even for production of such sheets of up to 40 mil thickness.
In current commercial practice, low density polyethylene (LDPE) in the form of biaxially oriented films is the most widely used material for the production of plastic bags and shipping sacks. However, LDPE film has relatively low tear strength. Isotactic poly-1-butene and its copolymers with small amounts of other monomers, commonly referred to as polybutylene homopolymers and copolymers, are superior to LDPE in respect to tear strength. This makes it possible to produce articles of a given strength from films of significantly lower gauge when using polybutylene rather than LDPE.
Blends of copolymers of butene-1 with small amounts of ethylene, hereafter referred to as "butene-1-ethylene copolymers" or "PBE," are products of commerce which have been employed for production of film by the blown-bubble extrusion process. In order to be able to take advantage of the inherently greater strength of polybutylene or PBE films compared to LPDE, it is necessary to modify some of the properties of polybutylene. For example, heat seals between layers of unmodified and uncompounded butene-1 polymers have an unsatisfactory seal strength. Incorporating 1 to 5 percent of LDPE into polybutylene, as disclosed in U.S. Pat. No. 3,634,551, improves heat seal strength somewhat. However, blends of butene-1 polymers and LDPE form films which can only be heat sealed over a relatively narrow range of temperatures.
It has recently been discovered by one of us that a blend of butene-1-ethylene copolymer and about 2 to 9% by weight of polypropylene (PP) may be used for the production of film which has an excellent overall balance of properties, including satisfactory heat sealing properties. This is disclosed and claimed in co-pending U.S. application Ser. No. 174,482, filed Aug. 1, 1980.
One of the significant properties of polymers if they are to be commercially useful for conversion to film by the blown bubble extrusion process is the ability to be processed at high line speeds. Line speed in the blown bubble extrusion process is established by balancing a number of factors, including the following: equipment parameters, such as design of the extruder, including extruder screw and air cooling ring, die diameter, die gap, die land length, etc.; polymer properties, such as the throughput rate at which extruded polymer acquires sufficient melt strength to hold the air pressure of the bubble; and process variables, such as extruder temperature and speed, temperature and rate of chilled air, air pressure in the bubble, etc.
One of the disadvantages of the film-forming compositions of butene-1 polymers with LDPE is that they exhibit a relatively lower processing speed in the blown-bubble extrusion process, compared to LDPE.
The compositions of Ser. No. 174,482 exhibit a significant improvement in maximum permissible line speed, compared with blends of LDPE with the same butene-1 polymer and compared with commercially available prior polybutylene compositions for film. However, a still further improvement in line speed was desired.